The present invention relates to a flat image display device in which electrons emitted from a plurality of cathodes disposed on a cathode board impinge on a phosphor screen coated on an inner surface of a front glass case to display an image.
FIG. 4 is a cross-sectional view schematically showing a conventional flat image display device. As shown in FIG. 4, the conventional image display device comprises a front glass case 21 having a phosphor screen 22 on an inner surface thereof and a rear case 23. The front glass case 21 and the rear case 23 are hermetically sealed by frit glass at a sealing portion 25. Within an airtight chamber 24 are provided a cathode board 26 having cathodes facing the phosphor screen 22 for emitting electrons and a collector electrode 27 for collecting electrons emitted from the cathodes. As shown in FIG. 4, the electrode board 26 is supported by a plurality of support columns 28 fixed to the inner surface of the rear case 23 to face the phosphor screen 22.
FIG. 5 is an enlarged cross sectional view schematically showing a broken line part 30 of FIG. 4. In FIG. 5, a reference numeral 31 denotes a cathode (for instance, a conical cathode) for emitting electrons. A plurality of cathodes are orderly arranged in matrix form so that the cathodes correspond to the phosphor dots composing the phosphor surface 22. In FIG. 5, a reference numeral 32 denotes a cathode electrode for applying a voltage to the cathodes 41, a reference numeral 33 denotes an insulating layer, and a reference numeral 34 denotes a gate electrode. Although the cathodes 31 in FIG. 5 are field emission type, they may be replaced with thermionic emission type cathodes.
In the above-described image display device, the electrons are emitted from the desired cathodes 31 when a predetermined negative voltage is applied to the cathode electrode 32 and a predetermined positive voltage is applied to the gate electrode 34. The emitted electrons are converged by electrostatic lens effect of the penetrating hole 27a formed in the collector electrode 27, and impinge on a metal back layer (not shown) provided on the phosphor surface 22 and to which a high voltage (e.g., +10kV) is applied. As a result, the phosphor dots of the phosphor screen 32 emit light to form an image. At this time, a considerable amount of heat is generated due to the electron emission in case of the field emission type or current flowing wiring an resistance in case of thermionic emission type.
However, in the above-described conventional image display device, since a high vacuum is maintained in the picture tube, there is no heat dissipation such as convection. Therefore, considerable heat is accumulated in the cathode board 6 to make it high temperature. The thermal expansion of the cathode board 6 makes the problems that a relationship between the electron emission points in the cathode 31 and pixels of the phosphor surface 22 is changed. As a result, a positional relationship between the cathodes 41 of the cathode board 36 and the phosphor dots of the phosphor screen 32 is changed, so the electrons emitted from the cathode electrodes 41 cannot impinge on the adequate phosphor dots, making it impossible to form an image of high quality. Further, in case of too excessive thermal expansion of the cathode board 6, it may be broken.